Linear refrigeration tunnels of this general type are widely known and are sold commercially as, e.g., Cardox ULTRAFREEZE.RTM., L'Air Liquide ZIPFREEZE.RTM., Koach IMMERSION PLUS.TM., Agefko Mehrlagentunnel CFM.TM., and Rommenholler ETAGENFROSTER TE.TM..
Known multi-tier linear refrigeration tunnels are typically composed of an insulated enclosure through which the various products to be refrigerated are transported by two or more, but typically three, parallel and superposed stainless steel or food grade plastic endless conveyor belts of selected width from an entry end to an exit end.
The known linear refrigeration tunnels may be of modular construction (i.e., the equipment may be composed of two or more adjacent and connected modular sections forming the previously described insulated enclosure). Such modular construction enables simple installation and simple size increase or decrease. Because such modular construction usually provides modular sections with horizontally hinged side doors that can be opened either manually (e.g., in the ULTRAFREEZE.RTM.device disclosed in U.S. Pat. Nos. 3,841,109 and 3,879,954) or hydraulically (e.g., in the devices disclosed in U.S. Pat. Nos. 4,580,413; 3,813,895; and 3,892,104), the modular construction enables simple and thorough washdown (because there are no hidden corners), easy and quick access to desired locations, and fast solutions to production incidents provided some basic safety mechanisms are installed, as they typically are.
Linear refrigeration tunnels may also be of nonmodular construction -- i.e., they are available in a few different sizes as set by the manufacture. Typically, such equipment is composed of one flat bottom insulation panel on which an upper U-shaped insulation wall rests during production. For washdowns, troubleshooting, and solutions to production incidents, the entire upper insulation is hydraulically raised. Typically, the raising operation is only possible when the equipment has warmed up. The disadvantages of the lack of modularity are obvious from contrasting the descriptions of the advantages of modularity in the previous paragraph. The advantage of lack of modularity is a usually lower manufacturing cost.
The known refrigeration tunnels achieve a refrigeration effect by creating a temperature driving force field by maintaining a given temperature level or profile in the cold gas atmosphere of the enclosure (using the previously mentioned refrigeration sources and suitable refrigeration medium delivery apparatus) and by creating a heat transfer coefficient field (by blowing cold gas on the surface of the products to be refrigerated using fans or other suitable means, by blowing cold gas jets on the surfaces of the products using the kinetic energy from the expansion of a high pressure liquid cryogen such as carbon dioxide, or by depositing the condensed phase of a liquid cryogen on the surface of the products using nozzles or other suitable means, but typically by fans or other suitable circulation means possibly combined with nozzles or other suitable injection means).
Since the production capacity, for a given usable belt area and for a given product, is directly dependent on the surfacic heat flux (which is the temperature driving force times the heat transfer coefficient -- i.e., .phi.=h (Tw=Ta)), optimizing production capacity requires (1) optimizing the temperature driving force through maintaining the lowest possible temperature within the enclosure and (2) optimizing the heat transfer coefficient, but under the constraint that the efficiency of the process be as high as possible. Due to that constraint, temperatures within the insulated enclosures of tunnels using liquid cryogens are usually regulated above their lowest possible levels (i.e., the saturation temperature of the cryogens at atmospheric pressure) in order to avoid accumulation of the condensed phase of the cryogens, and, if applicable, to make use of the sensible heat of the vaporized cryogens. Due to that constraint, the number of nozzles is limited to a certain maximum number which is a function of nozzle characteristics and of freezer characteristics.
Due to the above listed limitations on temperature levels within the freezer enclosure and on the use of liquid cryogen injection nozzles, optimizing the production capacity of the refrigeration tunnels requires optimizing the flow created by the fans.
The prior art shows two types of ventilation in multi-tier linear refrigeration tunnels. One type (shown in U.S. Pat. Nos. 3,841,109; 3,879,954; and 3,892,104) is directly derived from the single-tier linear refrigeration tunnel technology which uses top-mounted fan motors driving horizontal fan propellers located above the upper tier to circulate the cold gas within the enclosure. The other type (e.g., L'Air Liquide ZIPFREEZE.RTM. three-tier refrigerators and the device disclosed in U.S. Pat. No. 3,708,995) broke away from the single tier linear freezer technology and use side-mounted fan motors driving vertical fan propellers located between the tier assembly and the lateral walls of the tunnel (instead of top-mounted fans) to circulate the cold gas.
The known types of multi-tier linear refrigeration tunnels simultaneously exhibited advantages and drawbacks.
The top-mounted fans version ensures a very high, nearly uniform (with blades of diameter nearly equal to the usable width of the belt) forced gas convection heat transfer coefficient on the upper conveyor. The forced gas convection heat transfer coefficient is only limited by the weight and shape of the products to be refrigerated, since increasing the rotation speed and/or the pitch of the blades of the fans will increase the coefficient until the flow created by the fans blows the products off the conveyor (which explains why some refrigeration tunnels, such as the ULTRAFREEZE.RTM., use variable fan speed control). However, the circulation of the cold gas created by the top-mounted fans is moderate at best on the lower tiers, since the lower tiers are separated from the fans by a nearly solid plane obstruction composed of both the product-carrying portion of and the return portion of the endless conveyor belts and of the products resting on the belts.
The side-mounted fans versions yield heat transfer processes with a better uniformity between tiers. However, the heat transfer process on the upper tier created by the fans is weaker than the heat transfer process on the upper tier created by the top-mounted fans version, since the fan delivery is now divided between two or more, but typically three, tiers. Also, the heat transfer process created by the side-mounted fans is not uniform across the width of the various conveyor belts, since the products on the outer edges of the belts have some lateral surface directly exposed to the flow created by the fans, while the remaining lateral surface is shielded from the flow by the product itself, and since the products on the outer edges of the belts are directly exposed to the flow created by the fans, while the following products, along the width of the belts) are increasingly shielded from the flow.
Also, the side-mounted fans versions require some structural modifications to the refrigeration tunnel design which have undesirable consequences. The fans are mounted either on the insulated enclosure with the motors in the ambient atmosphere or on a fixed frame within the insulated enclosure with the motors in the cold gas atmosphere. With the outer-side-mounted fans version, it is no longer possible to open manually the horizontally hinged side doors of the modular sections of a linear freezer initially designed in that fashion (e.g., the ULTRAFREEZE.RTM.). This is so first because of the increased weight of the side doors, second because of safety considerations, and third because of increased design complexity. Outer-side-mounted fans are typically found on multi-tier refrigeration tunnels, the insulated enclosures of which consist of a flat bed and an upper enclosure, the enclosure consisting of a top and lateral insulation panels, the flat bed and upper enclosure being of the non-modular type, and the upper enclosure being hydraulically lifted for washdown or repairs or after production incidents. With the inner-side-mounted fans versions, modular construction is still possible, but the volume of the insulated enclosure is increased, the fan motors give off heat in the cold gas atmosphere, the specifications on the fan motors are more severe, and correct washdown of such linear freezers is complicated due to the additional area and due to the additional recesses suitable for bacterial development.
In summary, it is the opinion of the inventors that prior-art multi-tier refrigeration tunnels offer the choices among:
(a) multi-tier refrigeration tunnels with top-side-mounted fans which can be of modular construction, thereby providing simplicity in installation, in size increase or decrease, ease in use, cleaning, and resolving production incidents; and which provide a very fast and nearly uniform heat transfer on the upper tier, but which provide only moderate to very slow heat transfer on the lower tiers; or
(b) multi-tier refrigeration tunnels with inner-side-mounted fans which can be of modular construction, thereby providing simplicity in installation, in size increase or decrease, possible ease in use and in resolving production incidents; and which may provide uniform heat transfer process between tiers. However, such freezers do not provide a uniform heat transfer across the width of the belts, they do not provide a very fast and uniform heat transfer on the upper tier, they are harder to clean, they require fan motors rated for very low temperatures, they require a greater insulated volume, they have higher steady state heat losses because of the fan motors, which cause heat losses to the cold atmosphere within the insulated volume; or
(c) multi-tier refrigeration tunnels with outer-side-mounted fans which may provide a uniform heat transfer process between tiers. However, they cannot be of the modular type without a significant increase in design complexity, they do not provide a very fast and nearly uniform heat transfer on the upper tier, and they do not provide a uniform heat transfer across the width of the belts.